1. Field of the Invention
The invention relates to semiconductor wafer characterization equipment and, more particularly, to techniques for automatically locating particles on a semiconductor wafer using a high-magnification imaging system.
2. Description of the Background Art
Semiconductor wafer characterization equipment typically includes a high-magnification imaging system such as a scanning electron microscope (SEM) coupled to an energy dispersive x-ray (EDX) detector. Such an imaging system, when used to scan a semiconductor wafer, provides information regarding particles and anomalies on the surface of the wafer. The combination of a SEM and an EDX within a common unit is generally known as a SEM/EDX unit. Such a SEM unit is Model Number JSM-IC848A manufactured by Geol and an illustrative EDX unit is Model Number Delta IV manufactured by Kevex.
A SEM/EDX unit provides, at a high magnification level, information concerning particle shape, morphology and chemical composition. Such information is useful for determining root cause contamination sources within semiconductor wafer processing systems. Such contamination relates directly to device yield variations during processing of semiconductor wafers. Consequently, effectively determining and eliminating root cause contamination sources is extremely important to the semiconductor industry.
In order to gather such information, however, it is necessary to define in unambiguous terms the position of any arbitrary point on the wafer such that the imaging system can repeatably locate that point. As such, the imaging system is used to locate anomalies and contaminant particles on the wafer surface. Analysis of the anomalies and particles provides contamination information that is used to eliminate the source of the contaminants.
However, because the particle dimensions are on the order of 0.1-1.0 mm, imaging system magnification must be on the order of 200 to 2000 times such that a particle can be observed on the imaging system display. If the imaging system is a SEM, then the imaging system display is typically a cathode ray tube (CRT). At such high magnification levels, only a small portion of the wafer (e.g., a 50 .mu.m by 50 .mu.m area) is visible on the imaging system display at any one time. Thus, the particle positions must be accurately known before the high-magnification imaging system can be used to view the particles.
Because the image viewing area in a high-magnification imaging system is relatively small, a high-magnification imaging system is generally considered inappropriate for providing a general overview of the surface of a semiconductor wafer. As such, other devices have been developed for this purpose and a two stage process for locating and analyzing particles is generally employed. In the first stage, a laser scanning device raster scans the wafer surface with a laser beam to locate most of the particles on the wafer surface. An example of such a device is a Tencor SurfScan 6200, manufactured by Tencor Instruments. The laser scanner creates a laser scan map of the coordinates of the wafer features and concomitant particles. This laser scan map uses, of course, the coordinate system of the laser scanning device to identify the location of surface features and particles. The manner in which the laser beam is scattered from the wafer surface features and particles yields signals from which estimated particle positions in terms of x and y coordinates can be determined. However, because the scattering mechanism is not completely understood, the signals are of little help in identifying the type, chemical composition, and possible source of contaminant particles. This specific information can only be obtained with the aid of a high magnification imaging system such as an SEM/EDX unit.
A critical aspect of this two-stage particle analysis technique is that the coordinate system used in the laser scanning device must be transformed to the coordinate system used in the high-magnification imaging system. Since the wafer is physically moved from the laser scanner to the imaging system, there is no way to guarantee that the coordinate system used in the laser scanner will apply when the wafer is moved to the imaging device. The wafer may be inadvertently rotated and/or origin coordinates of the two systems may not be the same. Therefore, the particle coordinates obtained from the laser scanner must be transformed to corresponding coordinates used in the imaging system such that the particles can be found and analyzed. To further complicate matters, the imaging system coordinate system which is generally calibrated relative to a wafer support such as a SEM stage is not the same as the wafer coordinate system. The wafer coordinate system is defined relative to the geometry of each wafer. Depending upon the wafer geometry, the wafer coordinate system may be offset and rotated from the imaging system coordinate system.
To provide a common coordinate system for the imaging system, the laser scanner, and the semiconductor wafer, the physical geometry of the wafer must be mapped onto the fixed coordinate system of the imaging system. The physical geometry of the wafer includes locations of particles upon the wafer surface, a location of a flat or notch on the edge of the wafer indicating wafer orientation, and the location of the center of the wafer. This physical geometry can be used to define a wafer coordinate system.
One technique used to provide a common coordinate system for notched wafers is disclosed in U.S. Pat. No. 5,381,004, issued Jan. 10, 1995, and herein incorporated by reference. This patent discloses a manual technique for determining a coordinate system for a SEM unit and transforming that coordinate system to a laser scanner coordinate system. In particular, the SEM unit coordinate system is determined by a SEM unit user manually aligning the "cross-hairs" on the SEM unit display with a plurality of wafer edge locations. At each location, the user notes the coordinates of the locations in the SEM unit coordinate system. These edge location coordinates are used to determine coordinates for a center of the wafer.
Additionally, the SEM unit user then locates a notch on the edge of the wafer and, by aligning the cross hairs of the display with the edges of the notch, determines coordinate locations for points within the notch. These points are used to determine a composite notch point that provides a single coordinate for the notch location. The center location of the wafer relative to the origin of the laser scanner coordinate system provides an offset for the coordinate system transformation. Furthermore, the position of the notch relative to its former position in the laser scanner coordinate system provides a measure of the rotation of the wafer relative to the laser scanner coordinate system. Using this offset and rotation measure, a coordinate system transformation can be derived that transforms the SEM unit coordinate system to the laser scanner coordinate system. Since the intent of the method disclosed in the '004 patent is to transform the SEM coordinate system to the laser scanner coordinate system, this patent does not specifically disclose a process for formally generating a wafer coordinate system that can be transformed to any other reference coordinate system.
In general, transformation between two coordinate systems can be completely defined by an offset and rotation angle. That is to say, the transformation from one coordinate system to another can be considered to include a linear movement of the x and y axes so that the new origin assumes a position displaced from the old origin, together with a rotation of the axes about the origin to a new angular orientation. Such coordinate transformations are well known, and simple equations for performing them may be found in almost any basic text on linear algebra, coordinate geometry, or related subject matter. For example, transformations are completely defined in a text by John J. Craig entitled "Introduction to Robotics: Mechanics and Control," 2nd edition, published by Addison-Wesley Publishing Company, Inc. (1989), and specifically on pages 25-30. The transformation equations can also be found in "Elementary Linear Algebra," by Howard Anton, pp. 229-30, published by Anton Textbooks, Inc. (1987).
A significant disadvantage of a manual coordinate system alignment method is that establishing the coordinate system takes an inordinate amount of time (on the order of one hour). As such, for each semiconductor wafer that is to be characterized within the SEM unit, one hour must be spent to align each semiconductor wafer, just to establish a wafer coordinate system in which characterization can be accomplished. Additionally, since a manual approach requires a SEM operator to visually align cross hairs on the SEM display with the edges of the wafer, an error can easily be made. Such an error would result in an erroneous coordinate system transformation and ultimately in an erroneous wafer characterization of the particles on the surface thereof.
Therefore, a need exists in the art for an automated method of quickly and accurately establishing a wafer coordinate system within a high-magnification imaging system.